In the field of this invention it is known that in CDMA systems, such as UMTS, particularly UTRA TDD mode, the data part of a burst is spread with a channelisation code that can have different spreading factors, i.e., different numbers of elements in the code. In UTRA TDD, the spreading factors that may be used for uplink physical channels are 1, 2, 4, 8 and 16.
For each physical channel an individual minimum spreading factor SFmin is transmitted by means of the higher layers to the user equipment (UE), e.g. a mobile telephone, i.e. the UE is allocated a spreading factor. The UE determines or is currently using a transport format combination (TFC), comprising for example the type of forward error correction, the power etc. for its uplink transmission. In UTRA TDD, for example, the UE then has two options with respect to the spreading factor it has been allocated:                1. The UE uses the spreading factor SFmin, independent of the current TFC.        2. The UE autonomously increases the spreading factor depending on the current TFC (e.g. if the UE decides it does not have sufficient power to use a spreading factor as low as the allocated spreading factor, it may increase the spreading factor, i.e. use a new spreading factor in its uplink transmission). This invention relates to this second option.        
In UTRA TDD mode the UE includes in its uplink burst an item known as a Transport Format Combination Indicator (TFCI) which informs the receiving communication unit, e.g. a Node B, of the TFC being used i.e. the TFCI data bits are used to inform the receiving end which transport channels are active for the current Radio Frame; this information also allows the receiving end to determine the spreading factor allocated/employed. An uplink burst (405) in UTRA TDD mode is as shown in FIG. 1, and as known from the technical specification ‘3GPP TS25.221’ (available from the website www.3gpp.org) of the 3rd Generation Partnership Project, where the TFCI bits (410A, 410B) are inserted either side of the midamble (415), (Note TPC (Transmit Power Control) bits (420) are also included as are data symbols (425A, 425B)). In the uplink of UTRA TDD mode, the TFCI bits are usually encoded producing a TFCI codeword, the TFCI codeword and TPC bits are always spread at spreading factor 16 irrespective of the spreading in the data portions of the burst.
When the UE autonomously changes the spreading factor, it is only allowed to do so in certain ways effectively governed by rules, which in UTRA TDD specify that the spreading factor may only be increased by moving the channelisation code along the lower branch of the allowed orthogonal variable spreading factor (OVSF) sub tree.
The OVSF tree (505) is shown in FIG. 2, where CSFk is the channelisation code, k is the code number, and SF is the spreading factor. Let ko and SFo be the original code number and spreading factor respectively, such that the associated channelisation code is given by Coo. If the UE selects a new spreading factor, SFn, then the position of the new channelisation code in the OVSF sub tree is given by
      k    n    =                    SF        n                    SF        o              ×          k      o      
Suppose a UE is allocated the following channelisation code C43, but decides to change autonomously the spreading factor to SFn=16. According to the above equation, the position of the new channelisation code is given by
      k    n    =            16      4        ×    3  where the new channelisation code is defined by C1612. By inspection of the OVSF tree shown in FIG. 2, we see that C1612 belongs to the lower branch of the C43 sub tree.
Considering the uplink burst in UTRA TDD mode, on the receive side, the Node B is unaware of the fact that the UE has changed spreading factor. However, there is a unique association between TFCI and spreading factor. Since the TFCI bits are always spread using the channelisation code in the lowest branch of the OVSF sub tree, the receiving end knows a priori the TFCI channelisation code. Therefore, by first detecting and then decoding the TFCI codeword, the receiving end can determine SFn. Once SFn is known, it can calculate the position of the new channelisation code Cnn from the original channelisation code C00 using the equation
      k    n    =                    SF        n                    SF        o              ×          k      o      
Thereafter, the burst will need to be processed in a manner taking account of the new spreading factor.
A need therefore arises for a CDMA receiver adapted to accommodate the possibility of the spreading factor having been changed by the UE.